1. Field of the Invention
This invention relates to semiconductor devices, and more particularly to methods for attaching a semiconductor die to a substrate (commonly referred to as "die attach").
2. Description of the Prior Art
Methods for attaching semiconductor dice to substrates are well known in the prior art. As shown on FIG. 1, a semiconductor die 13, which may be, for example, either a transistor or a complex integrated circuit device, often measuring approximately 15 mils in thickness (1 mil=0.001 inch) and in excess of 100 mils (0.1 inches) on each side, is attached to substrate surface 19 located within opening 12 of semiconductor package 10. Substrate 19 comprises, for example a ceramic package. As shown in FIG. 1, a plurality of electrically conductive leads 11 are affixed to package 10. After the semiconductor die 13 is attached to substrate surface 19, a plurality of electrical "bonding wires", typically comprising aluminum or gold wire having a diameter of approximately 1 mil (0.001 inches), are attached between specified locations (often referred to as "bonding pads") on the surface of semiconductor die 13 and selected leads 11, thus providing electrical connections between regions of semiconductor die 13 and external circuitry (not shown) connected to leads 11. These wires are typically connected to the bonding pads and leads 11 by mechanical and thermal welding.
A cross-sectional view of semiconductor die 13 attached to substrate surface 19 is shown in FIG. 2. As shown in FIG. 2, an adhesive die attach material 14 is located between semiconductor 13 and substrate 19.
A good die attach is essential for several reasons. First, the die attach must affix the die 13 firmly to substrate 19 during the life of the device. Should the die 13 become separated from substrate 19, the aforementioned bonding wires may break or short, thus causing an electrical failure of the device. That die 13 remain firmly affixed to substrate 19 is important regardless of the use to which the device is to be put, but is much more so in military and aerospace applications, where the package 10 is sometimes subjected to very high stresses and impact forces. For this reason, stringent die attach requirements are provided for semiconductor devices to be used in military applications. These requirements are specified in MIL-STD-750B (transistors) and MIL-STD-883B (integrated circuits). Among these requirements are the minimum forces (i.e. often referred to as the "die shear strength") required to cause the die 13 to shear from substrate 19.
Secondly, the die attach must provide adequate thermal conductivity between semiconductor die 13 and substrate 19 to dissipate the oftentimes large amounts of heat generated by semiconductor die 13. This thermal conductivity is typically measured as the rise in temperature of the die per watt dissipated by the die (i.e., .degree.C./watt). For many transistors, such heat or power dissipation requirements are within the range of approximately 0.25 to 5.0 watts; for many high speed, complex integrated circuits, the power dissipation requirements are within the range of approximately 2.0 to 3.0 watts. Since the operation of the device is in major part dependent on its temperature, the die attach must provide a sufficiently low thermal resistance to the flow of heat generated by semiconductor die 13 to substrate 19, thereby maintaining the temperature of the die 13 within its specified operating temperature range. Substrate 19 in turn conducts heat to the package exterior which in turn conducts heat to the ambient surrounding package 10 (FIG. 1).
Thirdly, the die attach must oftentimes provide a good electrical connection between the backside of semiconductor die 13 and substrate 19. In these cases, it is important to have good electrical conductivity from the backside of the die to an electrically conductive cavity in the packages. This is achieved, for example, by connecting one of the leads 11 to the metallization in the cavity where the die attach is made. In order to provide backside electrical connection, the die attach medium must be electrically conductive.
Accordingly, a semiconductor device is considered adequately attached to the base of the package cavity when it passes a combination of mechanical, thermal and electrical requirements which reflect the expected operating conditions of the semiconductor device. There must be little or no mechanical stress resulting from the die attach process, and little or no stress concentrations from the die attach medium, as these stresses could result in cracking or chipping of the die. Failure of the die attachment often causes the device to cease functioning. Mechanical failure of the die attachment results in a lifted die, which may result in failure of bond wires and failure of backside electrical contact, as well as poor thermal operating characteristics.
One die attach method is gold-silicon eutectic die attach, which utilizes a gold-silicon alloy to weld the semiconductor die to the substrate. To accomplish this, the substrate needs to be treated with a material which will allow a weldment to be made to the substrate, and the backside of the die needs to have an appropriate finish and chemical composition. For the substrate, which is usually alumina or beryllia, a thin layer of gold and glass is fired onto the die attach cavity base, resulting in a surface rich in metallic gold to which a weld may be made. In order to provide sufficient gold to alloy to the die and form a strong weld to the base, gold-silicon eutectic preforms are added and allowed to melt on the treated surface of the base prior to placing the silicon die in the cavity. After the die is placed in the cavity, it is mechanically moved parallel to the surface and allowed to wet to the molten alloy. After the die is wet and positioned, it is left at the elevated temperature for several seconds to complete the process and thus finish the die attach operation. Two difficulties with this process are that a significant amount of time is required to manually complete the die attach operation, and the use of significant amounts of precious metal is quite expensive.
In order to overcome the high expense associated with the use of gold in die attach operations, die attach utilizing organic and inorganic high temperature glues, such as glues containing polyimide and polyimide-amide compounds have been used. These glues are temperature resistant to the extent they can withstand temperatures of approximately 450.degree. C. for periods up to 15 minutes, but tend to degrade rapidly with time at elevated temperatures. EPO-TEK P-10, sold by Epoxy Technology, Inc. of Billerica, MA is typical of the silver filled polyimides/amides used for die attach. By utilizing silver filled polyimides in place of gold, the expenses of the die attach operation is significantly reduced. However, such polyimides used in the die attach operation are organic compounds which decompose to yield moisture at high temperatures such as those generated during hermetic sealing of packages after die attach.
A graphical representation of the relative die shear strength as a function of thickness of the polyimide die attach adhesive is shown in FIG. 3a.
Another adhesive suitable for use as a die attach adhesive is silver filled glass, such as the paste-like product number JMI4600 manufactured and sold by Johnson Mathey Inc. of San Diego. JMI4600 comprises approximately, by weight, 16% glass, 64% silver and 19% organic binders and solvent. When utilizing silver filled glass as a die attach adhesive, a small amount of the silver filled glass is applied to a substrate and a semiconductor die placed thereon. The substrate, die, and die attach adhesive are then elevated to approximately 70.degree. C. in order to cause the evaporation of the solvents contained within the silver filled glass adhesive. Burning off of the organic solvents and sintering is then achieved by raising the temperature to approximately 350.degree. C. Conductive lead wires are then attached or "bonded" between leads on the package or substrate and selected locations on the surface of the die. The package lid is placed on and sealed to the substrate during a seal process at approximately 470.degree. C. If desired, the solvent evaporation step, and the glass melting step of the die attach operation when utilizing silver filled glass, is accomplished by passing the package, die, and adhesive combination through a furnace on a conveyer belt. The combination structure passes through different zones of the furnace thus providing the desired time and temperature "profile" required to first perform the solvent evaporation step, and secondly the glass melting step of the die attach operation. Such a temperature profile used for JMI4600 is shown in FIG. 6. Although terpineol, having a boiling point in excess of 120.degree. C., is used as the solvent in JMI4600, the solvent evaporation step is performed at a temperature less than 120.degree. C. to provide a uniform solvent evaporation. The use of glass as a die attach adhesive overcomes the disadvantages of the polyimide die attach adhesive in that glass is inorganic and does not give off appreciable moisture. The use of silver in the glass provides good thermal and electrical conductivity of the die attach adhesive. However, the relative die shear strength of the glass die attach adhesive is highly susceptible to variations in the glass die attach adhesive thickness. As shown in FIG. 3b, the relative die shear strength when utilizing silver filled glass die attach adhesive is maximum with a glass thickness of approximately 0.002-0.006 inches. For glass thicknesses of less than 0.002 or greater than approximately 0.006 inches, the relative die shear strength decreases quite rapidly, although sufficient die shear strength is achieved if the thickness of the glass die attach adhesive is maintained within a range of approximately 0.002-0.006 inches. For this reason, when utilizing glass die attach adhesive, the glass thickness must be controlled within a very narrow range centered at approximately 0.004 inches. Such accurate control of the thickness of the glass die attach adhesive over a narrow range of thickness is very difficult and thus glass die attach is not widely used at this time.
Other alloys used for attaching a semiconductor die to package substrates include a zinc-aluminum alloy as described in U.S. Pat. No. 3,956,821 issued May 18, 1976.
Inorganic solder processes include the use of silver/palladium alloys which can achieve good adhesion when used with various glass and glass/metal alloy systems. Often the inorganic systems include rather exotic metallic depositions on the wafer backside and some type of metal/glass or tungsten deposition on the substrate base.